I now hand the conference over to Dan Clifford, Managing Director. Please go ahead.
Thanks, Harmony. Good morning, and thank you for your time this morning, particularly on such short notice. I have Ian Poole and Peter Trout with me on the call. Through the call, we'll be referring to the presentation that was loaded up on the ASX site this morning, called Dargues Life of Mine Update. Just getting us started, I think, you know, business, the way we look at business is about leaning in, getting our eyes on it, and stepping forward. You know, where we're aware of risk sometimes, there is a step back to be taken. Disappointingly for us this morning, we have a step back, and we need to flag to the market today, an end-of-year non-cash impairment at the Dargues asset.
To take us through that, we'll run through the technical drivers leading to a range of approximately AUD 60 million-AUD 80 million on a post-tax basis, the timeline of planning, drilling, and modeling, and the priorities for adapting of the asset on the basis of these outcomes. Let's move to slide five. Important just for context for the discussion this morning. Let's have a look over our shoulder at what the investment thesis for Dargues was about. It was an on-strategy gold lean to our portfolio. What we were particularly aiming at was a rebalance or a reweight towards gold in the portfolio, the introduction of an asset that actually improved our group cost structure. What we could see during this was a resource that was mainly constrained by lack of drilling.
We could see improving grades at depths and multiple extensional opportunities of the ore body. Let's move over to slide six. With the ownership, I think everyone will be well aware that we immediately deployed particularly underground drilling and also a little bit later on surface drilling to be covering both infill from underground, pardon me, and exploration and infill from the surface and gain a better understanding of the asset. As we worked through these drilling programs, and particularly with the results that have been coming in over the immediate past, complexity become really evident. Really with the ore body, and particularly below the initial two mining levels that we acquired, resulted in this increased complexity.
One key area of the ore body is the key contributor to an actual downgrading in that area, but also then resultant across the resource with a grade reduction of approximately 15%. That's led to the expected impairment range of AUD 60 million-AUD 80 million. Just for us to better understand the technical drivers of that, I'll hand over to Peter to talk through the approach that we've taken. Thanks, Peter.
Thanks, Dan. I'll start on slide seven. Just from some of Dan's comments there. We've drawn on some new data to update our interpretation of the Dargues deposit. With that, the tonnage and grade estimates and Life of Mine plan. The updated Life of Mine plan we've developed just recently shows a loss of mineralized tonnage in some of the highest grade portions of deposit. That's been partly offset by the addition of lower grade material at depths. Slide seven provides more details in the long section through the mine design as it stands at present. By way of context, it's important to note that there's no reliable visual indicator of gold mineralization. In other words, it's not possible to see where the gold is present in the rock at Dargues. Infill drilling, face sampling, and mapping are essential tools for our understanding of the deposit.
Mineralized zones at Dargues were originally interpreted and modeled to be near vertical, with some variability in their strike orientation. This original geological interpretation and modeling was supported by development, face mapping, and sampling in the upper levels of the mine prior to acquisition, and were used for the 2021 Life of Mine plan, Mineral Reserves, Ore Reserve update, and Production Target. As Dan mentioned, post-acquisition, we've undertaken two underground drill campaigns to infill gaps in the mineral resource model. This infill drilling has progressed from top to bottom as the underground drill sites have become available from mining. The long section on slide seven shows the 305 and 425 levels, which are the platforms for the phase two infill drilling that's currently underway.
As a reference point, we're now developing to the ore body on 440 level, which is below the elevation of these two drill platforms. Turning to slide eight, we see here some cross-sections through the deposit showing changes to the geological model over the past year. The data sources columns show the more comprehensive data set that's been collected and used for interpretation and modeling for the 2022 life of mine plan. Compared to the previous wide-spaced surface drill holes, the infill drilling has provided tighter definition of the mineralized zones, especially at depth. The information from the 50 new drill holes has resulted in a drill spacing of 25 meters or less in the active mining areas today. This provides us with greater confidence in the mine design and our production schedules.
The latest geological model also shows some localized areas where the grade is significantly lower, where the mineralization is discontinuous or a different orientation to the original interpretation. Slide 9 provides sections through one of the key mineralized zones, Zone 8B. It's in this lobe where we've seen the greatest impact on the new data and modeling. I will point out that this lobe contains a substantial portion of the gold that was to be mined in the 2021 Life of Mine plan. You'll see circled at the top of the long sections, the upper western portion of this high-grade zone, which was interpreted originally as being continuous between surface drill holes.
Our images are a plan view that illustrate the extension of the lode along strike in the geological model, and you can see how that's varied over the course of the past year based on the recent drilling and what we're seeing now in the life of mine plan. Additional drilling and mapping has disproved the original interpretation in that upper western area, and that's led to the removal of a significant tonnage. More importantly, grades are above 8 grams per tonne gold. I will point out that the latest interpretation is not all negative. Long sections also show that the recent drilling has extended the interpreted mineralization at depth, which has added tonnage to the 2022 mine plan at a lower grade. This additional mineralization has partly offset some of the material that was removed from the previous model.
With those points in mind, I'll hand back to Dan, who will discuss the next steps.
Just to put a few touches to the finish there. As these are beyond the current mining horizons for this year, there is minimal impact on the FY 2022 targets. We will revise over the course and as results come in, particularly on the back of outstanding core logging, processing and assays. We're revising the model and production targets for the asset over the next few months. We're aiming for release of that in July in the next financial year.
Final actual value, as everyone I think will be able to understand, as we're flagging a range here is gonna be dependent on price decks, tax treatment, the outstanding results from cores and assays coming in for it to sign off, and ultimate board approval with the FY 2022 full year results. If we move on to slide 11. I'm not gonna dwell on the timetable, but what I'd like to point out is that we have quite a constant flow of business planning through the business, starting in quarter one with risk, moving through strategy, LOM and budget at the end of the year to set and ready for the next year. The drilling programs aren't actually timed with this planning.
It's actually the drilling programs came in on an ASAP basis, as soon as we could get the resources in the rigs, and also importantly, underground real estate to position the rigs to get the best angles for the drilling for the confidence levels. The fact that the incoming results fell over the back of the long period is a timing coincidence, but it actually put us in quite a good shape or in good position to be able to quantify these ranges really quickly and this close. That's why we're here this morning. Let's move on to slide 13. The key priorities. I'll summarize this.
The key priorities really are about completing as much as we can of the LOM drill out, the finalization of these results and backlog of core and assay, and really settle the asset down by adapting it for the outcomes of these results and maximizing the outcomes with the hand we have. 3,600-odd meters of logging to be completed, assays coming in on both infill and exploration holes, and that enables us and will inform the estimations and therefore the revised LOM plan Production Target in preparation for the FY 2022 and more on Production Target for July this year. Moving on to slide 14. Then back to the investment thesis of an under-drilled resource, particularly the extensions. We still firmly believe those opportunities remain and that part of the thesis remains intact.
We have both surface and underground exploration, particularly on body extension, and we're gonna start moving to the regional near mine targeting that we've assessed on the operation. In addition to that, the permitting that's required to extend the life of the asset is continuing. We do consider that future set to be still very prospective, and the asset continuing to move forward. Thanks, Harmony. I think it's best that we move on to questions now.
Thank you. If you wish to ask a question, please press star one on your telephone and wait for your name to be announced. If you wish to cancel your request, please press star two. If you're on a speakerphone, please pick up the handset to ask your question. We'll pause a moment for questions to register. Once again, to ask a question, please press star one. Thank you. There are no questions at this time. I'll just hand back to Dan for closing remarks.
Look, I think it's pretty clear to say we've had a step back, but myself and the management team and the board still think the asset can and will still continue to step forward. We will be next in market with exploration update, Federation update on how project is progressing and also the quarterly results in the second half of April for the March quarter. Thank you again for your time on such short notice. We appreciate that, and we'll keep the market up to date with progress. Thanks very much.
Thank you. This does conclude our conference for today. Thank you for participating, you may now disconnect.